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THE AMERICAN NATURALIST [Vol. XLIV 



synezesis, that one may well ask whether synezesis may 

 not have a deeper significance than the mere apposition 

 of like chromosomes. In fact the process seems well 

 suited to bring into close and intimate fusion the pairs 

 of chromosomes instead of simple apposition as appears 

 in the sporadic instances cited above. The situation calls, 

 at least, for a suspension of judgment until we have more 

 evidence. 



The number of chromosomes in closely related forms 

 presents one of the most puzzling problems when we at- 

 tempt to apply the chromosome view to the facts of 

 heredity. The case of the thread worm of the horse, As- 

 caris megalocepliala, is the best-known case. In some 

 localities the worms have four chromosomes for their 

 full number, in other localities only two. The animals 

 are identical externally, and occasionally where both 

 forms exist crosses occur. In such hybrids three chro- 

 mosomes are found in the embryo, but unfortunately no 

 adult worms have as yet been seen with three chromo- 

 somes. Such a worm would offer an exceptional oppor- 

 tunity to study the reduction problem. In other groups 

 similar variations in numbers are known between closely 

 related species. For example, one of the phylloxerans 

 has 44 and another 12 chromosomes, yet the two species 

 differ only in minor points, and every structure in one 

 has its counterpart in the other. If the chromosomes are 

 the bearers of the hereditary characters how can such 

 facts be interpreted? 



If we think of each chromosome in the one species as 

 containing the unit characters of a leg, or a wing, or an 

 eye, how are the same characters distributed in the other 

 species? Evidently a complete redistribution of such 

 units must be conceived. If genetic continuity is to be 

 extended also to the origin in time of the unit characters 

 in species, it seems to me inconceivable that so vital a 

 question as the assortment of these characters should so 

 readily change in closely similar, and probably closely 

 related species. Difficult as it is to interpret this rela- 

 tion, the simplest view would be to assume that it makes 



